If we were to go out into space a few millions of miles from either
pole of the sun, and were endowed with wonderful keenness of vision,
we should perceive certain facts, viz: That space is frightfully
dark except when we look directly at some luminous body. There is
no air to bend the light out of its course, no clouds or other
objects to reflect it in a thousand directions. Every star is a
brilliant point, even in perpetual sunshine. The cold is frightful
beyond the endurance of our bodies. There is no sound of voice in
the absence of air, and conversation by means of vocal organs being
impossible, it must be carried on by means of mind communication.
We see below an unrevolving point on the sun that marks its pole.
Ranged round in order are the various planets, each with its axis
pointing in very nearly the same direction. All planets, except
possibly Venus, and all moons except those of Uranus and Neptune,
present their equators to the sun. The direction of orbital and
axial revolution seen from above the North Pole would be opposite
to that of the hands of a watch.

[Illustration: Fig. 38.--Orbits and Comparative Sizes of the Planets.]

The speed of this orbital revolution must be proportioned to the
distance from the sun. The attraction of the sun varies inversely
as the square of the distance. [Page 100] It holds a planet with a
certain power; one twice as far off, with one-fourth that power.
This attraction must be counterbalanced by centrifugal force; great
force from great speed when attraction is great, and small from less
[Page 101] speed when attractive power is diminished by distance.
Hence Mercury must go 29.5 miles per second--seventy times as fast
as a rifle-ball that goes two-fifths of a mile in a second--or be